Melting and pouring of ferrous metals
Domain 7 - Melting and pouring of ferrous metals
This domain tests furnace choice, cupola operation, charge materials, melt treatment, pouring temperature and ladle practice for cast iron and steel.
Furnaces and charges
The cupola is a continuous shaft furnace for cast iron; charge materials are pig iron, scrap, coke and limestone. Its zones are the well, combustion or tuyere zone, reducing zone, melting zone and preheating zone. Limestone forms slag and removes ash and sulphur. Induction and electric arc furnaces give closer composition and temperature control and are used for steel and high-grade iron.
Melt treatment and pouring
Inoculation with ferrosilicon controls graphite size in grey iron; magnesium treatment produces ductile iron; aluminium or ferrosilicon deoxidises steel. Pouring temperature must exceed the liquidus by enough superheat to fill the mould: grey iron is commonly poured at about 1300–1400 °C, steel at about 1550–1650 °C. Preheat ladles, skim slag, keep a steady stream, and keep the sprue full.
Worked example
A grey iron has a liquidus of about 1200 °C and needs 100 °C superheat at the mould. The ladle loses 50 °C between tap and pour. The required tapping temperature is 1200 + 100 + 50 = 1350 °C. If the iron is tapped at 1300 °C, the pour may be too cold, giving misruns or cold shuts.
Traps
Wet or oily scrap causes gas defects and explosions. Excess superheat increases shrinkage and burning of sand. Do not judge temperature by colour alone; use a pyrometer. Slag must be removed, not stirred back into the metal. Inoculant must be added late and mixed effectively.